Mosquitoes become infected with the virus when they bite birds that have the virus. This is why Harris County collects dead birds, so they can be tested for West Nile virus antibodies.

Where did the disease come from?

It's been around for a while - some scientists now believe it could have contributed to Alexander the Great's death. But the first cases did not occur in the U.S. until 1999. The disease then spread westward across the country in a few years.

Has it been this bad before?

Dallas is having an epidemic this summer, and a public health emergency has been declared because there have been more than 200 cases and 10 deaths. Houston and Harris County have seen a combined 20 cases. That's well below the more than 100 cases in 2002, when the virus was first coming into the area. There was another peak in 2005.

What is being done about the disease?

Harris County has an excellent surveillance program for these kinds of diseases - it began in 1965 after a bad St. Louis encephalitis outbreak. Along with dead birds, the county collects mosquitoes and tests for viruses. When "hot spots" of virus activity are discovered, the county performs aerial spraying. It also sprays storm drains where these mosquitoes lay their eggs.

Is spraying effective?

The director of Harris County's mosquito control program, Rudy Bueno, says Houston is not seeing the kind of outbreak Dallas is because of spraying activities. Robert Tesh, a University of Texas Medical Branch expert in viruses, said that may be the case, but the answer is not clear-cut. "You'd like to think that's the reason," Tesh said. "But to be honest, nobody really knows how effective the spraying is for these kinds of mosquitoes.''

How prevalent is the disease in humans?

There have been few studies of how common the virus is in humans. A study of homeless people in Harris County in 2004 found that 7 percent had already been infected. Similar studies of dogs have found more than 50 percent to have been infected. The CDC estimates, from blood bank data, that only about 1 percent of the total U.S. population has been infected with the disease.

Will I know if I become infected?

About one in 150 people infected with West Nile will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent.

Are there any effective treatments?

There are no federally approved vaccines, although a lot of research is being conducted to find one. There are also no effective anti-viral drugs to treat the class of viruses to which West Nile belongs, Flaviviruses. But Tesh noted that even if there were effective anti-viral drugs they wouldn't do that much good in most cases because by the time patients show serious symptoms the body has already developed antibodies. In these neuroinvasive cases the virus has already spread into the spinal fluid.

What mosquitoes carry the virus?

It's not the floodwater mosquito, which spikes in population after rainstorms and is more active during the daytime. Instead it's the Culex mosquito, which is more apt to bite at night, and which are tan-colored. Culex mosquitoes lay their eggs near stagnant water sources.

Should I be scared?

Not overly. The disease poses a significant threat to the elderly, those who are undergoing cancer therapy or have had an organ transplant, and who have compromised immune systems. They should avoid going outside without mosquito protection. Younger people are safe for the most part, but it's probably best to take precautions.